Natural killer T (NKT) cells have been implicated in a range of important immune surveillance mechanisms, such as host defense against external pathogens, immune tolerance and malignancy. NKT cells can be further divided into two subsets, so-named Type I and Type II. Type I NKT cells have received the most attention. These cells are also known as invariant NKT (iNKT) cells owing to their expression of an invariant a chain T cell receptor (TCR; Vα14-Jα18 chain in mice and Vα24-Jα18 chain in humans), which is paired with a more variable β chain. In contrast, Type II NKT cells have a diverse TCR repertoire and are less well defined, although a subset has been shown to be reactive to sulfatide. The iNKT cell TCR recognizes lipid antigens presented in the context of the non-polymorphic MHC class I-like protein, CD1d. The CD1d molecule has been shown to bind a range of dialkyl lipids and glycolipids and the ensuing iNKT cell TCR recognition of the CD1d-lipid complex leads to the rapid proliferation and release of a plethora of cytokines (both pro-inflammatory and regulatory). The activation of iNKT cells is an important step in ‘boosting’ adaptive immune responses through the activation and maturation of dendritic cells (DC) and B cells through CD40-CD40L interactions, and the activation of natural killer (NK) cells following interferon gamma (IFNγ) release. Since the structure of CD1d ligands has been shown to govern the released cytokine profile, the development of lipid molecules that promote the specific activation of iNKT cells, could find very useful application in the treatment of a wide range of disorders.
Of the range of lipids that bind to CD1d, the glycolipid α-galactosylceramide (α-GalCer) is one of the most potent. α-GalCer is a derivative of the agelasphins, which are naturally occurring glycolipids that were isolated from the marine sponge Agelas mauritianus. Recognition of the α-GalCer-CD1d complex by the iNKT cell TCR results in the secretion of a range of cytokines, and the initiation of a powerful immune response.
While α-GalCer remains one of the most potent iNKT cell agonists and has shown potential in the treatment of various conditions, it may prove difficult to use this molecule widely as a useful therapeutic agent, at least as a direct activator of iNKT cells: not only does α-GalCer-mediated iNKT cell activation lead to the secretion of both T helper Type 1 (Th1) (e.g. IFN-γ) and T helper Type 2 (Th2) (e.g. interleukin-4 (IL-4)) cytokines, and therefore a mixed immune response, but more importantly over-stimulation of iNKT cells, which can result in their entering a long-term anergic state, i.e. unresponsiveness to subsequent α-GalCer stimulation and preferential IL-4 production, which would be deleterious for long-term therapy. Loss of circulating levels of iNKT cells could represent a therapeutically significant limitation with iNKT-cell-based therapies if multi-dosing regimens are required. Thus, a need exists for other iNKT cell activators.